[X86] Fix a couple bugs in my recent changes to vXi1 insert_subvector lowering.

A couple places didn't use the same SDValue variables to connect everything all the way through.

I don't have a test case for a bug in insert into the lower bits of a non-zero, non-undef vector. Not sure the best way to create that. We don't create the case when lowering concat_vectors which is the main way to get insert_subvectors.

llvm-svn: 320790